DIFFERING STRUCTURAL STYLES OF HINTERLAND AND FORELAND THRUSTS: A CASE STUDY IN THE PANAMINT RANGE, CA

Hinterland thrust faults are fundamental components of Cordilleran orogenic systems; notwithstanding, foreland thrusts have received the lion’s share of investigative focus and inspired models that are widely promulgated to typify upper-crustal shortening. The structural style of hinterland thrusts, which develop near regions of arc magmatism with high geothermal gradients or involve deeper crust in metamorphic environments, may significantly differ from foreland thrusts in that the footwalls are weaker and collapse or are underthrust. Here we present thermochronology and thermometry data from the northern Panamint Range, CA, that elucidate the deformation conditions of hinterland rocks in the immediate backarc of the Jurassic-Cretaceous Cordilleran magmatic arc and constrain the cooling history of the Jurassic Panamint thrust sheet. Zircon (U-Th)/He (ZHe) thermochronometric cooling ages from the Panamint thrust sheet range from Late Jurassic to Paleogene, and cooling age-paleodepth trends reveal a slow, uniform exhumation rate of ca. 0.05 km/myr over this time interval. Raman spectroscopy of carbonaceous material (RSCM) thermometry data record the peak metamorphic temperatures experienced by rocks within and below the thrust sheet, and peak temperature-paleodepth trends define a prograde metamorphic field gradient that may correspond to the geothermal gradient during a phase of Cordilleran orogenesis. Furthermore, when Neogene rotation of the Panamint thrust is restored, the thrust trace and footwall stratigraphy dip ~35° west while hanging wall rocks have subhorizontal attitudes. We posit that the synkinematic exhumation rate of the Panamint thrust sheet, which is lower than published exhumation rates of foreland thrust sheets by over an order of magnitude, reflects a unique structural style in which hanging-wall rock uplift is limited, shortening is primarily accommodated by the underthrusting of footwall rocks, and long-term exhumation rates are controlled by isostatic uplift commensurate with crustal thickening. This model sharply contrasts with those of foreland-style fault-bend-folds, in which thrust sheets experience significant rock uplift during transport up ramps, and may be generally applicable to thrust systems located in hinterlands and adjacent to magmatic arcs.